Antenna with high scanning capacity

ABSTRACT

The invention concerns an antenna with high scanning capacity. The antenna comprises a panel ( 30 ) of static radiating elements which are controlled to transmit in variable directions relative to a direction ( 38 ) perpendicular to the plane of the panel. Reflectors ( 34, 44 ) amplify the scanning effected by the panel ( 30 ) of radiating elements. The reflectors ( 34, 44 ) are segments of paraboloids with the same axis ( 38 ) and the same focus ( 40 ), for example.

BACKGROUND OF THE INVENTION

The present invention relates to an antenna with high scanning capacity.It relates more particularly to an antenna for use in atelecommunications system, in particular a telecommunications systemusing satellites.

Antennas are frequently needed, in various applications, to receivesignals from a mobile source and/or to transmit signals to a mobilereceiver or target. Such transmit and/or receive antennas are usuallyactive antennas made up of stationary radiating elements in which thedirection of the radiation pattern can be varied by varying the phase ofthe signals feeding the radiating elements.

That technique cannot produce satisfactory radiation patterns for largesquint angles, i.e. for directions departing significantly from the meantransmit and/or receive direction.

A source or a receiver can also be tracked using a conventional antennamoved by a motor. That type of antenna with mechanically movableelements and a motor is not suitable for all applications. Inparticular, it is preferable to avoid the use of any such antenna inspace applications, for reasons of reliability, overall size, andweight.

The invention remedies those drawbacks. It provides an antenna with ahigh scanning capacity and with a satisfactory radiation pattern atlarge squint angles, and which does not require any moving parts.

The antenna of the invention comprises a set of static radiatingelements commanded to perform scanning and reflector means to amplifythe scanning angle of the radiating elements. The reflector meansinclude two reflectors having a common focus, the first reflectorreceiving the beam transmitted by the set of radiating elements and thesecond reflector receiving the beam reflected by the first reflector.

SUMMARY OF THE INVENTION

According to the invention, the focal length of the first reflector isgreater than the focal length of the second reflector so that the exitbeam of the antenna has an inclination to a predetermined directionwhich is greater than the inclination Θ relative to the given directionof the beam transmitted by the radiating elements.

The scanning angle of the radiating elements can therefore be reduced inproportion to the amplification provided by the reflector means. Thusthe radiating elements are not used for squint angles that are toolarge. Also, the constraints imposed on radiating elements to scan asmall angle are much less severe. In particular, the dimensions of thesystem are less restricted, which enables a sufficiently large pitch(distance between two adjacent radiating elements) to prevent arraylobes without compromising the propagation of the radiation.

The reflector means are in fact analogous to those usually employed toincrease the size of the beam, for example in Cassegrain antennas.However, with the invention, the reflector means are used in theopposite way to usual. In a Cassegrain antenna, an increase in the sizeof the beam corresponds to a reduction in the scanning angle.

In one embodiment of the invention, each reflector is a paraboloid, forexample. The scanning amplification gain depends on the ratio betweenthe focal lengths of the two reflectors.

This ratio is 4/1, for example.

The reflectors are disposed so that the output beam is not even partlymasked by the first reflector, i.e. the reflector receives the beam fromthe radiating elements directly.

A preferred application of the invention relates to an antenna forcommunicating with a plurality of sources or receivers in an extendedarea, communication having to remain confined within the area despitethe changing position of the antenna relative to the area.

This problem arises in particular in a telecommunications system using anetwork of satellites in low Earth orbit. A system of this kind hasalready been proposed for high bit rate communication betweenterrestrial mobile or fixed stations in a particular geographical areacovering several hundred kilometers. The altitude of the satellites isin the range from 1000 km to 1500 km.

In the above system, each satellite includes groups of transmit andreceive antennas and each group is dedicated to a given area. Thereceive antennas in each group receive signals from a station in thearea and the transmit antennas retransmit the received signals toanother station in the same area. The antennas of a particular grouppoint towards the area for all the time it remains within the field ofview of the satellite. Accordingly, for each satellite, a region of theEarth is divided into n areas, and as the satellite moves over a region,each area is allocated a group of transmit and receive antennas whichpoint towards it continuously.

While the satellite is moved over a region, which takes around twentyminutes, for example, assigning a single group of transmit and receiveantennas to an area avoids switching between antennas, which couldcompromise the speed or quality of communication.

The low altitude of the satellites also minimizes propagation times,which is beneficial for interactive communications, in particular inmultimedia applications.

Clearly, with the above telecommunications system it is preferable foran antenna intended for one area not to suffer from interference due tosignals from other areas and for it not to interfere with other areas.Also, the radiation pattern has a shape which varies with the positionof the satellite relative to the area. When the areas on the Earth areall circular, the antenna sees the area in the form of a circle when thesatellite is at the nadir of the area; in contrast, as the satellitemoves away from this position the antenna sees the area in the form ofan ellipse that is progressively flattened as the satellite approachesthe horizon.

BRIEF DESCRIPTION OF THE DRAWING

It has been found that an antenna in accordance with the invention inwhich the reflectors are paraboloids can match the trace of the patternon the ground to the position of the antenna relative to the areawithout it being necessary to modify the radiation pattern produced bythe radiating elements.

Also, the antenna has a high gain when the satellite is close to thehorizon relative to the area. This is when the distance from thesatellite to the area is the greatest; accordingly, the increasing gaincompensates for the increase in the distance, which is favorable tomaintaining calls.

In one embodiment, two antennas of the above type are used to track anarea, each antenna effecting an even smaller scan.

An antenna of the invention can be used to track more than one area, theradiating elements being able to receive signals from or send signals tomore than one area.

DETAILED DESCRIPTION OF THE INVENTION

Other features and advantages of the invention become apparent from thefollowing description of embodiments of the invention given withreference to the accompanying drawings, in which:

FIG. 1 is a diagram showing a telecommunications system linkingterrestrial mobile or fixed stations using a system of satellites;

FIG. 2 is a diagram showing a telecommunications system;

FIG. 3 is a sectional diagram of an antenna of the invention;

FIG. 4 is a sectional diagram of a variant antenna;

FIG. 5 is a diagram showing the region that the antenna shown in FIG. 4can cover;

FIG. 6 is a diagram showing two associated antennas covering all theareas shown in FIG. 6; and

FIG. 7 is a perspective diagram of an embodiment using associatedantennas.

The example of an antenna to be described is intended for atelecommunications system using a constellation of satellites in lowEarth orbit, approximately 1300 km above the surface 10 of the Earth.

The system must set up calls between users 12, 14, 16 and one or moreconnection stations 20 to which service providers such as databases areconnected (see FIG. 1). Calls between users are also set up via theconnection station 20.

These calls employ a satellite 22.

In the system, the satellite 22 can see a region 24 of the Earth at alltimes and this region is divided into areas 26 ₁, 26 ₂, . . . , 26 _(n)(see FIG. 2).

Each area 26 _(i) is in the form of a circle having a diameter ofapproximately 700 km. Each region 24 is defined by a cone 70 centered onthe satellite and having a cone angle determined by the altitude of thesatellite (see FIG. 1). A region is therefore that part of the Earthwhich is visible from the satellite. When the altitude of the satelliteis 1300 km, the cone angle is approximately 110°.

Terrestrial means are used for communication between areas, for examplecables between the connecting stations of the various areas that arepart of the same region or different regions.

The number and the disposition of the satellites are such that at anytime two or three satellites can be seen from an area 26 _(i). When anarea 26 _(i) leaves the field of view of the satellite assigned to callsin that area, there is therefore another satellite ready to take over,and switching from one satellite to the other is instantaneous.

However, such switching occurs only about once every twenty minutes. Inpractice this switching occurs when, for the area 26 _(i) in question,the elevation of the satellite drops below 20°.

As the satellite crosses a region 24, the antennas of the inventionalways point towards the same area or the same set of areas. They musttherefore have a high capacity for scanning or squinting.

To this end, as shown in FIG. 3, the antenna comprises a panel 30 ofradiating elements associated with a beam-forming network (not shown)controlling the phase of the signals feeding the radiating elements. Abeam 32 transmitted by the panel 30 is directed towards a firstreflector 34 having the form of a paraboloid with a circular cut-off.The reflector is part of an imaginary surface 36 whose axis 38, on whichthe focus 40 lies, is far away from the reflector 34.

The axis 38 is perpendicular to the plane of the panel 30.

The reflector 34 reflects the beam 42 towards a second reflector 44 onthe side of the axis 38 opposite to the reflector 34 and the panel 30.The reflector 44 is also part of an imaginary surface 46 in the plane ofFIG. 3, which is a parabola with the same focus 40 and the same axis 38as the parabola 36. The surface 46 is also a paraboloid.

The concave side of the reflector 44 faces towards the concave side ofthe reflector 34.

The focal length of the reflector 44 is one quarter that of thereflector 34, for example.

The axis 38 does not intersect the reflector 34 or 44. The edge 44 ₁ ofthe reflector 44 nearest the axis 38 is at a distance from that axissubstantially less than the distance from the corresponding edge 34 ₁ ofthe reflector 34 to the axis 38.

In the example shown in FIG. 3 the array 30 has a generally circularexterior shape with a diameter of approximately 30 cm (12λ) with 37radiating elements separated from each other by 42 mm (1.7λ), where λ isthe wavelength of the radiation.

Each of the reflectors has a circular cut-off. In this example, thediameter of the circle defining the reflector 34 is in the order of 28λ.The diameter of the circle defining the reflector 44 is in the order of30λ. The distance between the edge 34 ₁ and the axis 38 is 24λ and thedistance between the edge 44 ₁ of the reflector 44 and the axis 38 is4′.

When the array 30 transmits a beam 32 ₁ parallel to the axis 38, i.e.perpendicular to its plane, the beam reflected by the reflector 34 isfocused at the focus 40. The reflector 44 therefore reflects the beam 32₂ parallel to the axis 38, as represented by the beam 32 ₃.

If the array 30 transmits a beam 32 ₁ inclined at a relatively smallangle Θ to the axis 38, the beam 32 ₆ reflected by the reflector 34converges at a point 50 near the focus 40, and the beam 32 ₇ reflectedby the reflector 44 is inclined at an angle which is approximately ntimes the angle Θ, n being the ratio of the focal length f of thereflector 34 to the focal length f′ of the reflector 44. In the example,the ratio between the focal lengths being 4/1, the beam 32 ₇ is inclinedat an angle of 4Θ to the axis 38.

However, this amplification in the ratio of the focal lengths does notoccur for beams 32 ₁₀ transmitted by the array 30, which beams have alarge angle of inclination to the axis 38.

Accordingly, FIG. 3 shows that the beam 32 ₁₀ is reflected as a beam 32₁₁ by the reflector 34 and this beam converges at a point 52 far awayfrom the focus 40. The beam 32 ₁₁ is reflected by the reflector 44 as abeam 32 ₁₂.

For example, for a beam with azimuth φ=90° and inclination Θ of 4.5° tothe axis 38, i.e. to the normal to the plane of the array 30, the beam32 ₇, also with an azimuth of 90°, is inclined at 18° to the axis 38.This value is indeed 4Θ.

On the other hand, for an inclination, or squint, of −14° (beam 32 ₁₀),again with an azimuth of 90°, the beam 32 ₁₂ has an inclination of 38°to the axis 38, which is significantly less than four times theinclination of the beam 32 ₁₀. The azimuth of the beam 32 ₁₂ is also90°.

In the example, for an azimuth of 90°, the beam transmitted by the array30 can scan an angle Θ in the range from 4.5° to −14°. These limits areimposed, in the first instance, by geometry because the beam reflectedby the reflector 34 must reach the reflector 44 and the beam reflectedby the reflector 44 must not be masked by the reflector 34. Secondly,the radiation performance of the beams converging in front of the focus40 (in the direction of the exit beam) also limits the scan anglebecause, for these inclined beams, operation is far from nominal.

FIG. 4 relates to a variant of FIG. 3 in which the reflector 44′ isgenerally oval in shape, i.e. longer in one direction than in theorthogonal direction, and the reflector 34′ has a circular cut-off, likethe reflector 34.

The greatest dimension of the reflector 44′ is in the plane of symmetryperpendicular to the axis 38 common to the two paraboloids. In thisexample, this greatest dimension is approximately 48λ.

The other features are the same as in FIG. 3.

The geometry shown in FIG. 4 yields the same performance for an azimuthof 90° as the antenna shown in FIG. 3.

For a beam transmitted by the array 30 with an azimuth of 0°, and for aninclination Θ of −5° to the axis 38, the exit beam is inclined at −20°with an azimuth of 2.3°. For a squint Θ of −15° and an azimuth of 0°,the squint of the exit beam is −45° with an azimuth angle of 31.5°.

With this reflector, and for an azimuth of 90°, the squint of the beamtransmitted by the array 30 can be varied in the range from +4° to −14°in the plane containing the center of the array 30 and the axis 38 andin the range from +15° to −15° in the plane of symmetry.

With these squint angles the antenna cannot cover all of the region seenby the satellite but only the portion 80 of that region which is shadedin FIG. 5. The portion 80 represents approximately 60% of the region.

To be able to cover all of the region, a pair of antennas arranged asshown in FIG. 6 is used. In this example, one antenna 90 transmits moretowards the West and one antenna 92 transmits more towards the East.

The two antennas 90 and 92 are fastened to a plane support 94 whosenormal 96 is directed towards the center of the Earth. In other words,the axis 96 always points towards the point 100 in FIG. 5.

The antennas 90 and 92 transmit towards regions which are symmetricalabout the axis 102 (FIG. 5). Thus the antenna 90 transmits towards theregion 80 and the antenna 92 transmits towards the region symmetrical tothe region 80 about the axis 102. The axis 38 ₁ of the antenna 90 isinclined to the axis 96 so that it is directed towards an area 26 pcorresponding substantially to the center of the region 80 (see FIG. 5).The axis 38 ₂ of the antenna 92 is of course inclined symmetrically.

It should be noted that the same array of radiating elements 30 can beused to transmit a plurality of beams. In other words, the same array 30associated with the reflectors 34 and 44 or 34′ and 44′ can be used totransmit towards more than one area or to receive signals from more thanone area.

In the example shown in FIG. 7 a common support 94 carries two pairs ofantennas 90 ₁, 92 ₁ and 90 ₂, 92 ₂. Each antenna, for example theantenna 92 ₁, comprises two panels of radiating elements, a transmitpanel 30 ₁ and a receive panel 30 ₂.

It can be seen that in all the embodiments the gain is greater at thelimit of the region 24 than at the nadir. The limits of the regioncorrespond to the greatest inclinations, for which the area concerned ofthe exit reflector (radiating aperture) is greatest and therefore forwhich the resolution is the highest. This property is apparent in FIG. 3where it can be seen that the reflector 44 of the beam 32 ₁₂ correspondsto a larger area than the beam 32 ₃. In this way, for the areas with thegreatest inclination, i.e. those at the greatest distance, the increasein the gain compensates for the increase in the distance.

It has also been found that the shape of the trace on the ground matchesthe target area.

What is claimed is:
 1. An antenna comprising a set (30; 30 ₁, 30 ₂) ofstatic radiating elements commanded to transmit a beam in variabledirections relative to a given central direction and reflector means(34, 44; 34′, 44′) including two reflectors (34, 44; 34′, 44′) having acommon focus (40), the first reflector (34, 34′) receiving the beamtransmitted by the set of radiating elements and the second reflector(44; 44′) receiving the beam reflected by the first reflector,characterized in that the focal length of the first reflector (34, 34′)is greater than the focal length of the second reflector (44′, 44′) sothat the exit beam of the antenna has an inclination to a predetermineddirection (38) which is greater than the inclination Θ relative to thegiven direction (38) of the beam transmitted by the radiating elements(30).
 2. An antenna according to claim 1, characterized in that each ofthe reflectors (34, 44; 34′, 44′) is a segment of a paraboloid.
 3. Anantenna according to claim 1, characterized in that the two reflectorshave a common axis (38).
 4. An antenna according to claim 3,characterized in that the common axis (38) is in the central direction.5. An antenna according to any of claim 1, characterized in that atleast one reflector is defined by a substantially circular edge orcut-off.
 6. An antenna according to claim 1, characterized in that atleast one reflector is defined by an elongate edge or cut-off.
 7. Anantenna according to claim 1, characterized in that the set (30) ofradiating elements is commanded to radiate simultaneously towards aplurality of separate areas (26 ₁, 26 ₂, . . . ).
 8. An antennaaccording to claim 1, characterized in that it is oriented so that theradiating aperture is larger for pointing directions corresponding totargets (26) at the greatest distances than for nearer targets.
 9. Anantenna according to claim 1, characterized in that it includes a set oftransmit radiating elements (30 ₁) and a set of receive radiatingelements (30 ₂) which are associated with the same reflector means. 10.A set of at least two antennas each of which is an antenna according toany preceding claim, characterized in that the radiating elements andthe reflector means of both antennas are symmetrical about an axis (96)constituting a central aiming axis of the antenna.
 11. The use of anantenna according to any preceding claim to a telecommunications systemusing non-geostationary satellites, the antenna, mounted on a satellite,being commanded so that it always views the same area (26 _(i)) as thesatellite moves over a region (24) divided into a plurality of areas ofsubstantially the same shape and size.